Radio detection and ranging systems



March 13, 1956 E. H. ARMSTRONG E-r A1. 2,738,502

RADIO DETECTION AND RANGING SYSTEMS 9 Sheets-Sheet 2 Filed DeC. 30, 1947 uwkm SIS

.NRt

ATTORNEYS March 13, 1956 E. H. ARMSTRONG ET AL 2,738,502

RADIO DETECTION AND RANGING SYSTEMS 9 Sheets-Sheet 4 Filed DeC. 30, 1947 INVENTORS rdn/fn H Armstrong Joh/7 h. 190.5@ and Robe/"f t'. f/a/l By, M @Ww/5W? ATTORNEYS March 13, 1956 E. H. ARMSTRONG ETAL 2,738,502

RADIO DETECTION AND RANGING SYSTEMS 9 Sheets-Sheet 5 Filed Dec. 30, 1947 r lllllllllllllllllllllllllllllllll Il INVENToRs idw/'f7 /7.l Armsfanq ./0/7/7 H 605e and Robe/"f5 /fw/ ATTORNEY;5

March 13, 1956 E. H. ARMSTRONG ET AL 2,738,502

RADIO DETECTION AND HANGING SYSTEMS 9 Sheets-Sheet 6 Filed Dec. 30, 1947 S HR @k INVENTORS m WM r /m ym Amm H55. Mmm i@ March 13, 1956 E, H. ARMSTRONG ETAL RADIO DETECTION AND RANGING SYSTEMS Filed Dec. 50, 1947 WaW//ch Genera/Ur imi/er Amp//f/'er De/ay C /rCL//f 9 Sheets-Sheet 7 INVENTORS ATTORNEYS March 13, 1956 E. H. ARMSTRONG ETAL 2,738,502

RADIO DETECTION AND RANGING SYSTEMS 9 Sheets-Sheet 8 Filed Dec. 50, 194'? QSE@ SQ INVENTORS g s M y m o ,Y O mw u ra# A Aww@ H0 5f ZM wn@ fn March 13, 1956 E, H. ARMSTRONG ETAL 2,738,502

RADIO DETECTION AND RANGING SYSTEMS Filed Dec. 50, 1947 9 Sheets-Sheet 9 C 70 #amm/'mfr w w gif". ,g5 Gas g Msc/large /69 g Tube INVENTORS idw/'f7 /f ,4r/775170 70 Fece/Ver Joh/7b. 505e and q Robe/"f5 Hu// BYzeaf,

ATTORNEYS t se rates l ate' tlit 2,738,502 R'Aio4 DETECTION AND RANGING sYsTE-ixfi'sv This invention relates to a new form of radio detection' and ranging system for distant objects,'commonly as radar', whichl Vutilizes the principles` of frqency modulation. It has for its object theA provision of aunore sensitive and selective system whereby greater range: and freedomr from interference of various sorts is obtained. It has also for its object the provision of means for distinguishing between xed and moving targets and1 `for measuring the speed of a moving target relative to the location of the radar station andthe sense of the motion. Other objects Willappear hereinafter.

Referring to the figures which form a part' of this specication:

Figures 1,: II- and III- are graphs which illustrate the gen# eral theory of operation of the invention. Figsi IV, V and VI- show block diagrams of the transmitter, receiver and indicator, respectively,` as actually used in operative foms of the equipment. Figs. VIIa and VIII) and VIII are electrical diagramsV showing details of specific parts ofthe equipment for which a block diagram would not be suciently descriptive.

The principle of operation of the systemA and the diculties th'atit is intended toy overcome' will first be con- Sider'ed. Itis Well known that the ordinary typeA of radar system operates by emitting short pulsesv of radio fre'- queucy waves of a substantially constant frequency; and that the `distance to the target is determinedy by measurement ofv the' time 'taken for the pulse to travet to the target and return to its initial starting point. In order to obtain a precise definition of the time as well as 'for other reasons it is necessary to make the' duration of the pulses very short, usually a small number of microseconds or mllionth parts of a second in length# In order tov receive this sort of pulse without distorting y'its' shape it becomes necessary to make the band width of the receiver circuits extremely wide; This givesr rise to afligh levell of noisev voltages generated within the receivery by reason' ofl irregularitiesv in the electron ,emission of the vacuum tubes and by the thermal disturbances4 among the electrons iiowingv through the conductors'.- The energies of both'these sources of disturbance`s,'as is well known,- varies directly as the' band width of the receiver. In practice it has been found necessary to make use of al band width ranging from one half to several megacycles or'moe. As the' reflected echoes which are tobe' re` ceived are generally extremely weak, the requirement-f a highly sensitive receiver and a broad band are" contradictory. For this reason the average pulse radar system isflimited to thedetection of signals ofthe order of a microvolt across the input terminals of the ordinary' type receiver.

Inraccordance with the present-invention it is proposed t'o dispense with the constant frequency pulse system-,and todetermine the range by use of a variable frequency Vso modulated and sofreceived thati the final band width of the receiverl may be less than 100 cycles ini width.' a consequence ofthis narrow band ofadmittance, and-as a consequence also of the use of the liete'rodyne system of detection in the manner hereinafter described, it is possible to receive extremely weak signals. In practice it has been found possible to receive signals of 1K5@ microvolt across the receiver input terminals with corresponding-increase in range. In the method of operation employed the transmitter and receiver divide time equally. It is therefore possible to increase the range still further since the transmitter' can be made to radiate substantially greater power than it is possible todevelop with the pulse system.

The general theory of operation is shown in the curves of Fig. I. Curve 1 illustrates the form of Wave. actually transmitted and to' tix ideas it will be usefuly to assign numerical values to the wave shown. Assume flej quen'cy' of unmodulate'd emission of 110 megacycles.' Curve I represents a wave that is varied sinusoidallylat afrequency1 related numerically to the distance to target in a" mannerY which will appear hereinafter. For the purpose of this explanation we will assume it is modulatedy at a frequency of 1000 cycles per secondend that a frequency deviation of plus and minus 200 kc.'is established, As already stated, the transmitter andreceiver divide timel equally, s'o ,that as illustrated in under thepconditions assumed, the transmitter sends out a single complete cycle abc" of the audio frequencyv modulation'. This cyclel is completed in one one-thousandtlt partof a" second;` fcllowingwhich there is :i priodof'n emission foi" the same interval "de"when,l4 at; pointl the aforementioned transmission cycle recommencesif Consider' now" atarget fixed' in' space 93 niilfs" away from the transmitter. Since the speed of transmission is 186,000 miles per second of frequency r'n'odulated wave abc will arrive there 1/2000 of al second later andy a very small part of its energy will be reflectedand returned to: its starting'- point 1&0@ part of aj second after its arrival at the target point. As a consequence the first cycleof the re'iiected wave train arrives at the"transmitting ypoint M50@ of a second after it initially started outD and coincident with the ending of the audio cycle of the transmis"- sion` at point c. The received vva'vef` may be considered a's a minute replica of the originally transmitted*wave,`de layed in time by the duration of one cycle fmodulation; Itf`c'o'incide's therefore exactly in form with the wave which would` have been transmitted duringth'at interval were the'V transmitter inA operation. It is therefore possible-'fof thisl set of conditionsA and for no other set to` deriveffroi'n the' modulating system of the transmitter a heterdynilig current of exactly similar form" as` the Vreceivedv ir/ol and coinciding so -precisely in deviation that a vconstant frequency difference between the two caribe produc'edi For the purpose of this explanation possible'niultipl" responses are left out of consideration. For purposes of"`exainpl'e a` heterodyning frequency'y of S'Ofmegacy'cle's may be assumed in which case, underthe conditions given,y a* constant frequency of 30 megacycles will be produced duringr those intervals of 1A000 second duration when the receiver is in operation. Since when the conditionsy of the correct modulation frequency for atarget at a given distance are met, a constant frequency echo current results in the receiver,it is possible to make thev band width of the receiver much narrower than heretoforel consideredv possible with consequent reduction in theA level of tube and thermal noisedisturbances.

it will be observed that for targets located at distances other than 93 miles from the transmitter that theI echo will arrive either before the transmitter .cycle has ceased or afterward, depending on whether the target .isy closer or farther away. Under these conditions a constant fre-1 quencyisno longer produced in thereceiver buta variable one which may take the form shown in Fig. II. As a' consequence these currents cannot pass effectively through the narrow band of the receiver so that the radar sees only targets for which the ranging or modulation frequency is adjusted;

Consider now the principal object of the system which is the detection of moving targets, and for the purpose of illustration consider a plane at a distance of 93 miles moving toward the transmitter at a speed of 400 miles per hour. Under these conditions the received wave will not be a replica of the transmitted wave but will, in accordance with the well known principles of the Doppler effect, be increased in frequency by a small amount, the increase being given by the formula where V is the radial velocity of the target in feet per second, c is the velocity of the radio propagation in feet per second, and fc is the transmitter carrier frequency. The lDoppler shift in frequency is indicated in the curves of Fig. III and for the conditions given above is an approximate increase of 131.2 cycles per second. lf the target plane were flying in a direction away from the transmitter the echo frequency would be reduced by this amount. It therefore becomes possible to differentiate between fixed and moving targets by means of this sys tem, a matter of the utmost importance for any radar operation. It-also is possible to ascertain the speed of the plane with respect to the transmitter and to determine the sense or direction of flight instantaneously so that it is no longer necessary to wait until a plot taken over a period of time gives the vital information of impending attack.

In all of the foregoing explanations the fundamental principles of the operation only have been considered.

The practical application of these principles is attended by very great difficulties as will be realized when the stability and distortion requirements of equipment oper-v ating at 100 megacycles and capable of passing received signals through filters less than 100 cycles in width are considered. The magnitude of the problem will be clear from the fact that a frequency instability of less than one part in a million would cause the received wave to drift outside of the receiver band width and that distortion as low as one-tenth of one per cent between cycles means a resultant beat frequency likewise outside the band.

The solution of the problems involved here in themselves require the application of new principles of operation and it is now in order to consider this part of the invention. It is impractical to conduct the process of selection at 30 megacycles which is the beat frequency used in the theoretical explanation heretofore given and the operation must be transferred to some lower fre quency in order to maintain the stability that is required of the system.

Referring now to Figures IV, V and VI, which show a block diagram of the practical embodiment of the system, Fig. IV covers the essential parts of the transmitter, Fig. V most of the essential parts of the receiver, and Fig. VI the indicator. Because of the nature of the problems encountered and the means taken to secure their solution, the transmitter, receiver and indicator elements are not completely independent entities but are interlocked in ways which will appear hereinafter. This is particularly true of the receiver, whose heterodyning currents for the various frequency conversions are obtained from the frequency control and modulating system of the transmitter.V

Referring now to Fig. IV, 1 represents a crystal controlled oscillator; 2 a phase shift modulator, preferably balanced; 3, 4, 5 and 6, 7, 8 a series of multipliers, the two series having identical multiplication and feeding, respectively, converters 9 and 10. 11 represents a band pass filter.

12 represents a crystal controlled oscillator:

for setting the frequency of the transmitted wave, an 13 a tripler for supplying heterodyning currents to'the balanced converter 10.

14, 15, 16 are a frequency multiplier chain, the output of which is fed to two amplier tubes 17, 18, which are arranged to be keyed by biasing beyond cutoff to interrupt the transmission of signals in the manner heretoforereferred to.

19 represents a multiplier, 20l an amplifier, 21 a multiplier and 22 a series of amplifiers, including the power amplifier which excites the antenna 23 in the usual way.

24 represents a TR box, or transmit-receive switch, which will be described hereinafter.

All the multipliers and amplifiers in the 19 to 22 chain are biased beyond cutoff, so that when excitation is removed by the keying of the amplifiers 17, 18 their plate currents all drop to zero.

An examination of the arrangement of the transmitter 1 to 23 will show that it is essentially similar to theV two, channel type of phase-shift modulator described in Edwin H. Armstrongs U. S. Patent No. 2,290,159, with the/addition of a couple of stages of keyed amplifiers inserted atl a point, in this particular case, of one-ninth the frequency to be radiated.

26 represents the usual correction circuit which is employed with the phase shift type of frequency modulation system to secure uniform deviation as disclosed in Armstrong Patent #1,941,068, and 27 represents the source of modulating current. This modulating current consists of a variable frequency oscillator covering `the range of 400 to 20,000 cycles for the purpose of ranging on particular targets.

28 represents a series of delay circuits interposed between the range oscillator and the switch generator containing thev elements 29 to 39. The purpose of the delay circuits will be hereinafter described. The elements of the switch generator, whose purpose is to alternately activate and deactivate the transmitter and receiver in accordance with the modulation frequency of the oscillator 27 will be described in detail subsequently.

The receiving system shown in Fig. V is essentially a four intermediate frequency superheterodyne. 50 represents one or more stages of R. F. amplification at the received frequency; 51 the first converter or mixer; 52 and 53-two stages of amplification at the first I. F. frequency; 54 the second converter and 55, 56 amplifiers at the second intermediate frequency. 57 represents the third converter and 58 and 59 amplifiers for the third intermediate frequency and having essentially a wide band. 60 represents an alternate amplifier for this same frequency having a narrow band. This amplifier may be substituted for the broad band amplifier in the manner shown.

61 represents the fourth converter for heterodyning to the final frequency which is used in the indicator. This, as will appear hereinafter, is of the order of a few hundred cycles.

62 represents a mixer supplied with frequency modulated current from a point in the transmitter at the output of multiplier 16 and also with a multiple of the frequency determining crystal oscillator 12. 63 represents a multiplier and 64 an amplifier for supplying the current of multiplied frequency from the crystal 12 to the mixer 62.l 65 and 66 are multipliers of the output of the mixer 62, and 67 an amplifier for supplying the multiplied output to the mixer 51. Y

68 is a multiplier, 69 an amplifier, 70 a multiplier, 71 a multiplier and 72 an amplifier for furnishing the hetero dyning current to the second mixer 54. This is 4a xed frequency heterodyne derived from the fixed frequency crystal 12. f y 73 is an amplifier arranged to amplify the output of the multiplier yfor furnishing the heterodyning current of the third mixer s7.

83 represents affree oscillator and 84 anv amplifier for supplying: a'. vfixed frequencyf heterodyning currenttto'f the fourth orfla'st mixer 61'.

` 754-8-2and18'5 represent' n frequency controll system for the free oscillator 83 to maintain it ati a frequency difference of' a couple ofl hundred cycles'froma kreference frequency' which is: a multiple of the crystalV controlled frequency 12. Specifically, 74 represents a mixer" sup= plied with three times the crystal controlled frequencyof the oscillator 12 and withA the frequencyof. the free oscillator 83-via the amplifier 85'. 75- represents alowpassiilter for the output of the mixer 74, '.76 an amplifier, 77`and178limiters respectively, 79a ZOO-cycle balanced discriminator and 80V and 81 balancedk detectors. whose output controls the reactance tube 82.

86 represents a resistance-capacity network supplied witlrcurrentA from theA output ofi the range oscillator 27 and designed to passV current inversely as the frequency to the rectifier 87. The output of the rectifier S7 is apL plidtothe amplifiers-52' and 53 to control their amplification' in accordance with thei range at"wh`ich thesystem is'beingf operated.

It will'be observed that the R. F. amplifiers Fandfthe mixers'51` are arranged to be keyed by the outputfofiy the amplifier 39 in accordance with the operation of thefswitch generator.- Under some conditions itvmay also he'advisable to key an additional amplifier in one of the It F. stages; as, for example, the amplifier 52.

Although' conductors have notV generally been` designated by reference characters, not specifically referred' to in this description, the connections A, B, C, D, E-and'F havebeen' marked-in Figs. IV and V to facilitate; reading of the figures.

Fig; Vi indicates two waysV of presenting'the targetindications. 100 representsa low passi filterA which, for operation on approximately 100 megacyclesa'nd-'alplane speedof 400 miles'an hour, may pass up to 350:-cycles per second. i is a rejection filter having a rejection band centered, in the present instance, at'200 cycles-,z with arejection of plus or minus approximately 10 cycles. 102 represents an amplifier feeding a loudspeakerl and headphone system 104, 105 with its associated'amplifier 103 106'to 111 represents a set of band pass filters, col lectively extending from 50 to 350 cycles, and each ap# proximately 50 cycles wide. 112 represents a band pass filter centered at 200 cycles per second and coveringapproximately the same band width that the 101 rejection lter is designed for. 'It will be observed thatjthe band pass filter 112 is4 connected aheadl of the rejection filter 101-, whereas the band pass filters 106-111'are connected in-thecircuit after the rejection lter 101.

113` to 119" are cathode ray tubes connected, respectively, to the band pass filters 106 to 112'.' 120'to'12'6" are theordinary sweep oscillators for individually sweeping the cathode ray tubes 113 to 119.

The manner of operation of this system may be understood from the following descripti'on.- Assume thatit is'fdesired to range on a target 93 miles away.V The `range oscillator 27 is set at 1000'cycles and modulates the transmitter system 1-23 so as to transmit a frequency modulated wave at 110 megacycles having a sinusoidal deviation of plus and minus 200,000 cycles. This variation in frequency occurs, of course, 1000 times per second.

Also connected to the range oscillator is a switch'generator 29e-39, a device actuated by the'- modulating frequency so as to cut off the transmitter onl alternatey cycles of modulation in the manner indicated in Fig. I: Itis requiredy that the transmitter be keyed 500 times a' second from a source of 1000 cycle modulating current. This is accomplishedl by means of the differentiating circuit`f33 and the alternate cycle divider fli4l-3Sv which produce 500 keying pulses per second. These pulses Vare-applied through the amplifier 36 to the keying of the tubes17 andl18, the ampliers of'the 12.2 megacyc'le sfageof-tl'le transmitter.

The operation ofthe-divider systemuis somewhat critical With respeetito the amplitude' of the'v applied voltage' from the'ra'nging oscillator. To insure correct operationover wide? ranges` of frequency of this oscillator', limiter'sf 30 and-31'1 areinterposed between the ran'gingoscillator and thedivider input toA produce a square wave of constant amplitude. To insure that the keying of the.' transmitted wavel occurs at the desiredpoint'of the modulatingcy'cle; adjustable. delay circuits` 2S are introduced between'the range oscillator and thelimiter 30.

Coincidentally one phasey of the output of the divider 34L-35 operates to key the transmitter, the'other phase of the di-vider outputv operates to key the receive'r'in the opposite" sense.` This phase of theoutput' is'amplitied by theamplifiers37, 38' and 39 and' applied 'tothe grids-'of the radio frequency amplifiers 50- and the mixer tube-511 Turning. new to the'receiver', it will be` observedthat the: receiverwillV be activated. when thel'transmitter/'h'as ceas'etliradiating and will. be-irl condition to receive the reflectedechofwhichwill be a replca-of-'that transmitted in thel preceding modulatoncycl'e. 'The firstheterodyning currentl is arrangedtol deviate in frequency precisely the san'l'ef amount and in the same sense' as theprevio'u'sly transmitted wave, but at sorne substantially different frequcrr'cyfrom-it. In the present case this heterodyning frequency is177.9 megacycles. This frequency is obtained by'combiningin the mixer'62'two' currents derived from different'parts'of the transmitter. One is a variable fre# quency current at 12.2`megacycles having one-ninth-th'e deviationl lof. the 'transmitted wave, and the other is a' sevenfold'multiple iixed'frequency current of thecrystl controlled oscillator 12. The difference' frequency results ina variable frequency current of-8.6 6 mega'cycles having a freque lcy deviation-f one-ninth that radiatedv from the antenna. By-'multiplying the'output of the'mixer bythe' tripler @tand-66 a heterodyning current havingthefrei quired` characteristics is produced.

Under'such-circumstancesthe reected signal CD1-2,` as indicated in Fig. I hasta constant frequency difference withlrespecttto' the heterodyne. This difference underthe conditions 'set up is 32.1 megacycles. Pro-m thispoint on the signal is progressively heterodyned down in the second mixer 54 to 6.62 megacycles, the heterodyning cur# rent for this frequency beinglderived from the 50th muli tiple of the master'crystal oscillator 12; This 6.62`mega cycle current is heterodyned down in the third`mixer5'7 t0 1.53 megacycles by the heterodyning current which is the 10th multiple'of the frequency of the master crystal controlled oscillator 12. The- 1.53' megacycles' then undergoes th'e principal 'amplification in the system by the ampiiiers 5S, 59; 60 and is supplied tothe fourth mixer 6'1i At this point the explanation of the system will .be'

facilitated bypoiuting out thatthe'lastintermediate frequency-1.53 megacycles-is exactly three times the frequency ofthemaster crystal controlled oscillator 12 and that if the output ofthe tripler'l, which is 1.53 mega-r cycles, were used as the.V heterodyning current forj the' fourth mixer the 'output frequency of the mixer would be zero. It is likewise in point to observe-that any drift in frequency of the master crystal controlled oscillator 12, either up or down, will not disturb this vrelationshipi The final output of the mixer 61 would he zero frequency and would remain so, so lon-g as the targetvdoes not move; if the` output of the tripler 13" were used directly as the heterodyning current in the mixer 61. The condition of zero beat frequency will also hold for'all' deviations, as any change inthe swing of the transmitted Wave is exactly compensated for by a change in the deviation ofthe- 'rst heterodyning current. It will be observed, therefore, that by reason of this process or" coordination between the con; trol-offthe transmitted frequency; and the heterodyneof thel receiver it` is possible to'hold the'heterodyned output of the'receiver to a stabilityY not. heretoforeaccomplished.`

Assumenow that the targetv is notiixed, but'is movingr towardlthefstation at a speedf'off400 miles fperlhourr- Un :ler these circumstances and for the frequency of 110 megacycles the received frequency will not be a replica of the transmitted frequency but will be increased by approximately 131 cycles per second. Since all heterodyning currents are on the low side, each intermediate frequency will be increased by the same amount and the final output will be a current of 131 cycles interrupted for symmetrical periods of /ooo of a second 500 times per second. This current may be passed through a low pass filter such as 100 to remove the high frequency interruption rate, leaving a continuous frequency of 131 cycles per second, which may be indicated by either audible or visual means.

Assume now that the target is moving away from the station similarly at a speed of 400 miles per hour. Under these fconditions the received frequency will be reduced by approximately 131 cycles per second, and similarly, each intermediate frequency will be reduced by the same amount. The final output of the mixer 61 will again be 131 cycles; that is, the Doppler frequency indication for the same absolute speed toward or away from the station will be exactly the same; indication of the direction of flight is not instantaneous but must be determined by observing the change in range of the plane as indicated by the adjustment of the range oscillator 27.

It is now in order to describe the arrangement whereby the sense of the motion of the target with respect to the station can be immediately obtained. If instead of utilizing the output of the tripler 13 as the final heterodyning current a separate oscillator running at 1.53 megacycles plus 200 cycles is employed as the source of heterodyning current of the mixer 61, then a different situation will result. A target traveling toward the station at 400 miles per hour would produce an output frequency of 69 cycles, while a plane traveling at the same speed away from the station would produce a frequency of 331 cycles per second. These two alternative frequencies would pass respectively through the filters 106 and 111 and would be indicated by the respective corresponding cathode ray oscilloscopes 113 and 118, thereby enabling instantaneous indication of the direction of flight. Correspondingly lower speed planes would appear in their respective scopes. It will be seen that, depending on the scope in which the indication appears, it is possible to determine immediately whether a target is moving toward or away from the station, as well as its radial speed thereto.

It is pointed out that for the indication of a target the setting of the frequency of the range oscillator 27 requires careful adjustment when the frequency deviation of the system is large and more freedom of adjustment when the deviation is small. The accuracy of the indication of target range increases directly with the deviation of the system.

In the practical operation of this system it is, of course, of the utmost importance that the frequency of the oscillator Asupplying the last heterodyning current be maintained with great accuracy at a xed frequency difference withrespect to the master crystal oscillator. This can be be `done in a variety of ways, such as making oscillator 83 crystal controlled, or by holding its frequency by means of a reactance tube such as 82, whose operation is controlled in any one of the well known methods of frequency stabilizing. In the present system the output of the oscillator 83 is supplied to the mixer 74 in conjunction with the output of the tripler 13 to get a beat frequency of the desired amount, namely, 200 cycles, which is passed through limiters 77 and 78 and a discriminator detector system 79, 80, 81, in a manner well known in the art. The balanced output of the detectors 80, 81 control the reactance tube 82 to maintain the frequency difference of 200 cycles.

It is now in order to point out two principal advantages of the system, both of which arise by reason ofV the extreme stability obtained by anchoring transmitter and receiver controls to the same frequency controlling crystal. By reason of this stability it is possible to operate in a practical way with filters having an admittancejband width as low as 50 cycles in the manner shown. Filters having band widths no greater than 10 cycles have been successfully utilized.v

The importance of this narrow band width in securing great sensitivity is of fundamental importance. In receivers at megacycles it is possible to design radio frequency amplifiers so that the principal noise arising within the system will be the thermal noise of the antenna circuit. Under these conditions, for a band width of 50 cycles the thermal noise for a Sil-ohm impedance input circuit is less than 1/{100 of a microvolt. Where no external noise is encountered it thus becomes possible to receive signals slightly stronger than this, and in practice an indciated response on input signals of the order of $0 of a microvclt have been obtained.l This is, of course, a sensitivity far greater than any heretofore realized in radar operation.

In'order to realize these sensitivities in practice it is necessary to take extreme precautions to eliminate any frequency modulation other than that produced by the range oscillator and all amplitude modulation in the transmitted wave. These modulations may result either from improperly filtered power supplies or from vibration of various sorts. Blower motors in the high power part of the equipment have been found particularly objectionable. The result of any of these modulations is to produce spurious responses in the scopes, particularly in the presence of large fixed echoes as will be hereinafter described. The elimination of these disturbances must be effected through extreme measures not hitherto employed in ordinary radio practice.

The second advantage of this system is the elimination of permanent echoes or reflections from fixed targets which at ordinary ranges may be many times greater than the reflections from moving targets. These are returned at-the transmitted frequency and so give a beat frequency of 200 cycles per second.

For further dealing with these echoes a band rejection filter 101 of about 20 cycles total width and centered at 200 cycles per second is inserted after the detector, In view of the narrow band of rejection required, a high rejection ratio at 200 cycles with respect to the rest of the band may be obtained. This filter will be effective against fixed targets provided no amplitude or frequency modulation resulting from 60 cycle or 120 cycle power supply equipment has been superimposed upon the wave. If such modulation is present, then frequencies of 200 cycles plus and minus 60, 200 cycles plus and minus 120, etc., appear. These pass by the rejection filter and through the respective band pass filters 106 to 111, being indicated on their respective scopes. Hence, it is essential that no such spurious modulation be permitted on the transmitted wave.

Modulation due to vibration, instead of appearing as discrete frequencies in the indicator usually appears as a disturbance simultaneously present in all the scopes.

It is of interest to note that harmonic distortion emanat ing from the range oscillator or through overloading any of the tubes in the modulating system is not ordinarily serious for the reason that a complete cycle of modulaf tion is utilized, and due to the stability of the system these modulation cycles repeat themselves accurately. The above statement applies only when the range frequency of the range oscillator is not being changed as it is in the case when the system is searching.

A differentiation between fixed and moving targets of several hundred to one has been obtained in practice. Greater differentiation depends upon the precision with which the various precautions that have been above referred to are carried out.

It will, of course, be obvious that the band pass filters may be subdivided as finely as desired so as to more accurately ascertain the speed of the moving target. Howfever, dueto` the increase in transient response as the 9 j ilters become narrower, vthis results -in a reduction -oflthe Speed at y,which searchfcan-be carried o n.

Itfhas-been found in practice lthatechoes from objects intheimmediatefvicinityv of the station setup transient -IeSllQnses which are occasioned as the resultof thetranslrnitter. keying. 'Iwo means of combatting them may housed. `One is.torinterrupt the cycle at -the point where --the frequency isV changed most rapidly. The other is to arrange v4toactivate the receiver a short interval after .they :transmitter has been keyed oli. These two precautions result in. increasing the dierence in the freq ueDCy of the echo from that to which the receiver is tuned to respond at the moment of the activating of the amplifiers S and mixer 51. The point of interrup- ,tion..o ff`.the .cycle is determined by an adjustable delay circuit 28`whichcontrols the'tirniug of the divider circuit .and .by theinclusion ofa blanking device 3S consisting o ffan R`C' circuit which-acts to delay the opening ofthe receiver to-incomingsignals for a small vfraction of the Y.modillation cycle.

The third` advantage of the system which has been -mentioned'hefore Vbut not stressed, is the ability to radiate aiwave o f'greater-'power than it is possible to obtain with the pulse system. This is because the power that can .be .emitted by any formof system depends, among other things,upon plate dissipation and peak voltage which the tube will-stand, and the maximum'lament emission of which it is capable. For a given plate dissipation much more .average power can be obtained from the tube whichjis operatingat a'substantially uniform output; or, as in the ,present case, on a 50% duty cycle. ln thisy type of operation the limitations of peak voltage and y lar nent' emission apply to a Amuch less extent than with the. shortfpulse type transmitter. As a consequence, increased range .for this reason alone is obtainable.

A Referring now to Figures VIIrz and VIIb, there are ,illuratedfthe .details of the switching mechanism for alternatelvkeyingthe transmitter and the receiver so as to dividetirne on a 50% duty cycle basis between them. This keying system, which is operated by the ranging frequency oscillator, must meet' the condition that it perform itsswitching. operation once every full cycle of the ranging frequency. This is accomplished by providing a` divider circuit which is operated in .the manner hereinafter to bedescribed.

.'Referringnow tothe details of the system, 2 84 represents a'ftimedelayfnetwork whose input is connected to the y range Oscillator 27 o f iFigureIV and whose output is conn'ectedto. thelamplier29. 30 and 31 arelimiters .for thepnrposevof squaring up the sine wave output ofthe rangingjfrequency oscillator. 32 ,is an `amplifier for this square wave,` and' 155, Y156l is .a' condenser-resistance vcombinationlinthe'output ofthe amplifier 32 so :proportioned a' s' to" act as 'a diie'rentiating circuit 33. 157 and 158 are tlflbs With'their associated' circuits 34 and 35 arranged to act "as aquick acting trigger dividing .circuit whoseoutput islalsquarewaveof onefhalf the frequency of the square wave 'in the'output of the amplier 32.

Ihe'plate circuitsofftheftubes 157 and-158 are arranged in push-pulll operation, with the tube 157 operating through the amplifier 36 to key the transmitter and'the ,tube 158operating through a series of three amplifiers 3'1,v 39 and 16410 ykey the receiver. The two'amplier chainsare keyedrespectively bythe voltage-drop across .heload 159'and 160 ofthe tubes l157 and 158.

Reference has heretofore been made `to a method of c ombatting the echoes from objects in the `immediate vicinity of the station'byarranging `to delayA thefactivation of the receiver funtil a short interval after the transmitter vhasbeen keyed'o." This delay or blanking circuit 38 is :arranged'in the output of the voltage'amplier 37 and consists o f la condenser 16Sshunting.a series of resistances 166', 167"' an` d" 1f6. 8. Selection-of the amount'of delay desired is made .by .the switch 169. Reference alsohas been-made to the combatting'.- of nearby fixed eci 1cies=-.by the adjustment of the time of switching to coincide-:with the greatest rate of change of the transmitted frequency. The delay vcircuits ,28 are .for the purpose ofrmakingfthis adjustment.

Inorder to effectively utilize thesame antennaforhoth transmission `and/reception yit is necessary, y where -high power is involved, Vto furnishsome means of llimiting.the power which may ber lfed into the receiver input. The `apparatus used for y, this'purpose is-known asa "lRbox, and the technique of itsfuse has been thoroughly developed ,in the case of pulse systems. In the 4case'ofthe present system the problem Iismore diicultbecause .higher average `powers are .used .and vbecause `the almost l continuous nature of the ltransmission results in a serious heating problem in the device Lwhose change in `conductivity `is madeuse .of to .check the owof energy intontherreceiver.

Figure VIII ,illustrates a new :type ,o f TR boxwhich `has been effectively utilized. In the arrangementshowfn, 180, 181 and 182 illustrates the transmission linetrunring from .the transmitter to Vthe antenna. The- TR .box 183, 184.and- 185 is connected to the main transmission ,lineat point 181 at a point three-quarters wave length i(or any odd multiple of aquarter wave length)4 from the end of the transmitter coupling lloop. In accordance lwith standard practice the receiver which is connected-:as indicated is protected bythe quarterwave section 184 whichcontains a gas discharge tube :186 `in the position shown. Two additional `features includea capacity 187 for tuning-out .the reactance ogthebottornendof the :center conductor of-the .quarterwave section 184, ,and an inductance 411,8 8 forneutralizing the capacity ofthegas discharge tube-1.86 .and `the parallel .tuning .condenser\.189. To'balance .the TR boxlwithrespect to the tw owire-maintransmission line anlolute`r'.shie1d183l s .provided `for .the yquarter wave :section 184.

With this -arrangementit yis vpossible-to hold the voltage developed'across the receiver input `terminalsto 'a point where the voltage developed by the transmitter isinsuiicient to overcome the biasing ,off voltage applied by the keyerto ythegrids tof theradio frequency tubes. This device has operatedsuccessfully with average powers of the orderoffve kilowatts. Y

Itwill be understood'that both of these expedients, namely, -.the division oftime between vtransmitter and receiver vbythe keyingsystem and the TR box are'necessary .because `ofthefhighl power employed andthe use-of the ,sameantenna for both transmitter and receiver. Where ,high power isnotutilized or whereother means of protectingthe receiver from the transmitter are employed,

Ysuch asjhighlyf .directional .antennas properly spaced, these expedientsv may. be .dispensed Y with. Theoretically, this would make amoredesirable system-.as the'keyin'geunder ,certain .circumstances may-give rise to spurious indications. In practice, ...llowex'erg` the system of dividi-ngftime .will Vproljsably Continue to .be preferable.

In the operation of this system .there is -a certain dif .culty encountered that is peculiar-to the system. `AtparticularA ranges and radiatedv frequencies the reections from tixed-targets'havesidevband components or -transients caused bytthe-keying.ofithetransmitter which give rise to falseindications in vthe moving target indicators. In effect this vlresul'tsfin -a "blinde spot for certain rangesv of operation at a- 'giv'entransmitt'edfrequency-as it is-dii-clt tofdistingushthe movingtar'getindicationsfrom the false signals."

The'-diflicul'tyl can `beobviated by the selection of trans- -mit-tedffrequences 'for given ranges so that the principal transientsfall either higher or lower than the Doppler'or moving .target frequencies', to enable theirfrejec'tion by ltering means,v

We ,haved'escrib'edwhat we believe to be .the-'best embodiments' .oi-ouvrinvention. We do not"wish,y.how ever, "to, .be .confined` tothe embodiments -shown, -but -wh at the transmitted signal during alternate modulation cycles only to be radiated and for enabling the receiver to receive the reflected signal during other alternate modulation cycles only, said receiver including converters in which frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are combined with the rellected signal to produce a derived signal current of predetermined constant frequency when the period of the range oscillator is adjusted to be equal to that of the out and back travel of the signal.

2. In a radar system, a frequency modulation transmitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, and keying mechanism controlled by the range oscillator for causing the transmitted signal to be radiated during alternate modulation cycles only, and for enabling the receiver to receive the reflected signal during other alternate modulation cycles only, said receiver including converters in which frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are combined with the reflected signal to produce a derived signal current of predetermined constant frequency when the period of the range oscillator is adjusted to be equal to that of the out and back travel of the signal, and indicator means responsive to said derived signal current.

3. In a radar system, a frequency modulation transmitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, and keying mechanism controlled by the range oscillator for causing the transmitter signal during alternate modulation cycles to be radiated and for enabling the receiver to receive the reflected signal during the other alternate modulation cycles only, said receiver including converters in which frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are utilized for 'heterodyning the reflected signal and for deriving from that results from the Doppler effect, and frequency indicator means for showing the magnitude and sense of such change of frequency.

4. ln a radar system, a frequency modulation transmitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, a frequency modulating lrange oscillator adjustable in fre quency to effect a signal frequency swing of selected frequency, and keying mechanism controlled by the range oscillator for causing the transmitted signal during alter nate modulation cycles only to be radiated and for enl abling the receiver to receive the reflected signal during other alternate modulation cycles only, said receiver inl cluding converters in which frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are combinedv with the reflected signal to produce a derived signal current of predetermined constant frequency when the period of the range oscillator is adjusted to be equal to that of the out and back travel of the signal, and, when the signal is reflected from a moving target, for producing a derived signal whose frequency is a function of the change of frequency that results from the Doppler effect, indicator means comprising a group of sharply tuned indicators jointly covering the entire range of frequencies that may be so derived, a narrow band rejection filter in series with said group for excluding from said group the frequency, within the composite range of the group, that is derived when the target is stationary and the period of the signal swing coincides with the period of out and back transmission, and a narrow band-pass filter in series with the last mentioned indicator for passing only the narrow band of frequencies which is excluded from the group of indicators.

5. In a radar system, a frequency modulation trans mitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, a fre quency modulating, range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, and keying mechanism controlled by the range oscillator for causing the transmitted signal throughout alternate modulation cycles only to be radiated and for enabling the receiver to receive the reflected signal throughout the other alternate modulation cycles only, said receiver including converters in which varying frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are combined with the rellected signal of varying frequency and with constant frequencies to produce a derived audio frequency signal of constant frequency when the target is stationary and the period of the signal swing is adjusted to be equal to that of the out and back travel of the signal, and to produce other audio frequencies which are a function of the speed and direction of travel of the target when the target is moving, all of which derived frequencies lie within a narrow range of frequencies, an audible indicator responsive to said derived frequencies, and a filter for excluding from the indicator all frequencies lying outside said narrow range.

6. In a radar system, a frequency modulation transmitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, and keying mechanism controlled by the range oscillator for causing the transmitted signal throughout alternate modulation cycles only to be radiated and for enabling the receiver to receive the reflected signal throughout other alternate modulation cycles only, said receiver including converters in which varying frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are combined with the reilected signal of varying frequency, and with constant frequencies to produce a derived signal of constant frequency when the target is stationary and the period of the signal swing is adjusted to be equal to that of the out and back travel of the signal, and to produce other constant frequencies lying in a narrow band at one side of said derived frequency when the target is approaching and at the other side of said derived frequency when the target is receding.

7. In a radar system, a frequency modulation transmitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, and keying mechanism controlled by the range oscillator for causing the transmitted signal throughout alternate modulation cycles only to be radiated and for enabling the receiver to receive the rellected signal throughout the other alternate modulation cycles only, said receiver including converters in which varying frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal of varying frequency and with constant `frequencies are combined with the reflected signal to produce a derived signal of constant frequency when the target is stationary and the period of the signal swing is adjusted to be equal to that of the out and back travel of the signal, and to produce other frequencies lying in a narrow band at one side of said derived frequency when the target is approaching and at the other side of said derived frequency when the target is receding, an indicator mechanism responsive to said derived frequencies, and a filter for excluding from the indicator mechanism all frequencies lying outside the narrow range including said bands.

8. A radar system comprising, in combination, a frequency modulation transmitter and a receiver, said transmitter comprising a signal generating mechanism which includes frequency controlling oscillator means, a constantly operating frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, a common antenna for the transmitter and receiver, and keying mechanism controlled by the range oscillator for rendering the transmitter operative to radiate the modulated signal during alternate modulation cycles and inoperative to radiate the modulated signal during the other alternate modulation cycles, and also for rendering the receiver inoperative to receive the modulated signals from the antenna or the transmitter during the first mentioned alternate modulation cycles and operative to receive signals from the antenna and the transmitter during the second mentioned alternate modulation cycles, said receiver including converters in which frequencies derived from the frequency controlling oscillator means of the transmitter and from the modulated signal are combined with the reiiected signal to produce a derived signal current of predetermined constant frequency when the period of the range oscillator is adjusted to be equal to that of the out and back travel of the signal.

9. In a radar system comprising a transmitter and a receiver, in combination, a signal generating mechanism which includes frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, keying mechanisms controlled by the range oscillator for causing energy to be radiated during alternate modulation cycles and not to be radiated during the other alternate modulation cycles and an adjustable delay circuit interposed between the range oscillator and the keying mechanism for determining the exact point in the cycle at which the keying becomes effective.

10. In a radar system comprising a transmitter and a receiver, in combination, a signal generating mechanism which includes frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, keying mechanism controlled by the range oscillator for causing alternate modulation cycles to be radiated and the other alternate modulation cycles not to be radiated, and for rendering the receiver inoperative to receive retiected signals during the first mentioned alternate modulation cycles and operative to receive reflected signals during the second mentioned alternate modulation cycles, and a variable resistance delay mechanism embodied in said keying mechanism and settable to cause a delay of desired duration between the interruption of transmitting and the resumption of receiving.

11. In a radar system, a frequency modulation transmitter and a receiver, said transmitter comprising a signal generating mechanism which includes frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, keying mechanism controlled by the range oscillator for causing the signal to be radiated during alternate modulation cycles and not to be radiated during the other alternate modulation cycles, the receiver including mixers for heterodyning the reflected signal to produce a derived signal of predetermined constant frequency when the period of the range oscillator is adjusted to be equal to that of the out and back travel of the signal, the receiver also including amplifier mechanism controlled from the range oscillator of the transmitter, and a resistance-capacity circuit through which current from the range oscillator is made effective to control the amplifier mechanism, said resistance-capacity circuit having the property of passing current inversely as the frequency and hence in proportion to the target range, so that the controlled amplification will be varied in accordance with the target range.

12. In a radar system, the combination with a transmitter employing a frequency modulating range oscillator whose vfrequency is designed to be adjusted in inverse relation to the target range, of a receiver including amplifier mechanism controlled from the range oscillator, and a resistance-capacity circuit interposed between the range oscillator, and the amplifier mechanism, the resistancecapacity circuit having the property of passing current inversely as the frequency and hence in proportion to the target range, so that the output of the resistance-capacity circuit when rectified can be used to vary the amplification in accordance with the target range.

13. In a radar system comprising a transmitter and a receiver, in combination, a signal generating mechanism which includes frequency controlling oscillator means, a frequency modulating range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, mechanism controlled by the range oscillator for causing alternate modulation cycles to be radiated and the other alternate modulation cycles not to be radiated, and for rendering the receiver inoperative to receive reflected signals during the first mentioned alternate modulation cycles and operative to receive reflected signals during the second mentioned alternate modulation cycles, said mechanism comprising a switch generator which includes a differentiation circuit, and circuit means comprising a divider of the modulating frequency for applying energy derived from successive cycles of -the modulating frequency in alternation to grids of transmitter and receiver amplifiers.

14. A radar system comprising, a frequency modulation transmitter and a heterodyne receiver, said transmitter including frequency controlling oscillator means, and a frequency modulation range oscillator adjustable in frequency to effect a signal frequency swing of selected frequency, and said receiver including converter mechanism in which frequencies derived from the frequency controlling oscillator means of the transmitter and the modulated signal are combined with the reliected signal to produce a derived signal of constant frequency when the period of r the range oscillator is adjusted to be equal to that of the out and back travel of the signal.

References Cited in the file of this patent UNITED STATES PATENTS 1,585,591 Lowy May 18, 1926 2,301,929 Budenbom Nov. 17, 1942 2,407,644 Beniofr Sept. 17, 1946 2,412,161 Patterson Dec. 3, 1946 2,412,315 Brown Dec. 10, 1946 2,422,134 Sanders June 10, 1947 2,422,157 Wolff June l0, 1947 2,423,644 Evans July 8, 1947 2,424,854 Sanders July 29, 1947 2,433,669 Keister Dec. 30, 1947 2,444,388 De Vries June 29, 1948 2,485,583 Ginzton Oct. 25, 1949 

